1. Field of the Invention
This invention relates to an image display device provided with opposed substrates and a plurality of spacers located between the substrates.
2. Description of the Related Art
In recent years, various flat image display devices have been noticed as a next generation of lightweight, thin display devices to replace cathode-ray tubes (CRT's). A surface-conduction electron emission device (SED) has been developed as a kind of a field emission device (FED) that serves as a flat display device, for example.
This SED comprises a first substrate and a second substrate that are opposed to each other across a predetermined gap. These substrates have their respective peripheral portions joined together by a rectangular sidewall, thereby constituting a vacuum envelope. Three-color phosphor layers are formed on the inner surface of the first substrate. Arranged on the inner surface of the second substrate are a large number of electron emitting elements for use as electron sources, which correspond to pixels, individually, and excite the phosphor. Each electron emitting element is formed of an electron emitting portion, a pair of electrodes that apply voltage to the electron emitting portion, etc.
For the SED constructed in this manner, it is important to maintain a high degree of vacuum in a space between the first substrate and the second substrate, that is, in the vacuum envelope. If the degree of vacuum is low, the life performance of the electron emitting elements, and hence, the life performance of the device lower inevitably. In order to support an atmospheric load that acts the first and second substrates and maintain the gap between the substrates, in a device described in Jpn. Pat. Appln. KOKAI Publication No. 2001-272926, moreover, a number of plate-shaped or columnar spacers are arranged between the two substrates. In displaying an image, in the SED, an anode voltage is applied to the phosphor layers, and electron beams emitted from the electron emitting elements are accelerated by the anode voltage and collided with the phosphor layers, whereupon the phosphor glows and displays the image. In order to obtain practical display properties, the phosphor used should be one that is similar to that of a conventional cathode-ray tube, and the anode voltage should be set to several kV or more, preferably to 5 kV or more.
In the flat image display device described above, a high voltage of 5 kV or more is applied between a front substrate and a rear substrate, whereby the electron beams emitted from the electron emitting elements on the rear substrate are accelerated and delivered to the phosphor on the front substrate. Since the luminance of the displayed image depends on the accelerated voltage, a high accelerated voltage should preferably be applied. In the case where the high voltage is applied, however, gaps, if any, between the first substrate or the second substrate and the spacers may possibly cause a problem, such as disturbance of the electron beams attributable to electric field concentration or electric discharge in micro gaps. If any electric discharge occurs, the electron emitting elements, a phosphor screen, or a driver circuit may possibly be broken or degraded.
Accordingly, the respective heights of the spacers must be controlled with high accuracy such that errors are 1 μm or less, to eliminate the gaps. Since a number of spacers are provided between the first substrate and the second substrate, however, it is technically difficult to make the heights of all the spacers uniform, so that the manufacturing cost is high.